Space-time/space-frequency coding for multi-site and multi-beam transmission

ABSTRACT

The present invention relates to space-time or space-frequency coding in cellular systems. The same data is transmitted from different antennas with different coverage areas, corresponding to different cells. The different data streams have different parts of the space-time block codes applied. A mobile terminal can combine the different parts of the space-time block codes in different received signals. This provides better performance than the known techniques for single frequency networks. The invention can also be applied to antennas with different coverage areas from the same site, and different beams formed with antenna arrays.

FIELD OF THE INVENTION

The present invention relates to a method, system, transmitter andreceiver for using at least one of space-time and space-frequency codesin a cellular system. In a particular example, the present inventionrelates to a multi-site and multi-beam transmission system. Moreover,the present invention relates to a computer program product for carryingout the afore-mentioned method.

BACKGROUND OF THE INVENTION

One of the aims of future cellular wireless communication systems is toenhance the achievable data throughput to mobile terminals (MT) situatedclose to a cell edge. This is important as, assuming a reasonablyuniform distribution of MTs over the cell area, then a significantfraction of the MTs in a cell is close to its periphery. When the samecarrier frequency is re-used in neighboring cells, the signal from the‘wanted’ base station (BS) with which the MT is communicating isreceived at the cell edge at power levels similar to signals originatingfrom BSs in neighboring cells. Cell edge MTs therefore experience stronginterference in addition to low signal to noise ratio (SNR), which makesit difficult to achieve high data rates to these MTs.

In current third generation (3G) systems like the Universal MobileTelecommunications System (UMTS), macro-diversity and soft-handovertechniques are known. These allow simultaneous communication betweenmore than one BS and a MT in order to improve the link quality to MTs atthe boundaries between cells. In macro-diversity and soft-handover thesame data is transmitted to a MT from multiple BSs. This is achieved byhaving all transmissions effected on the same carrier frequency, anddiscriminating transmissions from different BSs based on their differentscrambling codes. The MT comprises a receiver arrangement for receivingmultiple (CDMA) signals simultaneously, i.e. it has multiple receivesignal paths (descrambling and decorrelation) and a combiner to combinethe despread symbol streams.

Macro-diversity techniques in 3G systems rely upon code divisionmultiple access (CDMA) techniques in order for a MT to receive a givendata stream from more than one BS simultaneously. However, each wanteddata stream causes interference to the reception of the other stream.

For future cellular systems, including 3^(rd) Generation PartnershipProject (3GPP) Long Term Evolution (LTE) and Wireless World InitiativeNew Radio (WINNER), multi-carrier (OFDM-based) transmission schemes arebeing proposed (at least for the downlink). Further, multiple access(sharing the time-frequency resources between MTs) is typicallyenvisaged to be based on allocating different groups of subcarriers todifferent MTs (orthogonal frequency division multiple access (OFDMA))rather than on CDMA. The macro-diversity techniques from 3G aretherefore not directly applicable to these future systems.

One favored approach for improving cell-edge performance in these futureOFDM-based cellular systems is to partition subcarriers between cells,wherein all cells may use all subcarriers at lower transmit powers. Thisgives coverage to the inner parts of the cell but does not reach thecell edges (and therefore does not cause interference to neighboringcells). For communication to MTs at the cell edges a BS then uses asubset of the total number of available subcarriers where the subsetsare defined to be mutually exclusive with the subsets employed inneighboring cells. This prevents higher power transmissions to cell-edgeMTs from causing high interference to cell-edge MTs in the neighboringcells.

This approach improves the inter-cell interference situation forcell-edge MTs at the expense of increasing the frequency re-use factor,which results in lower spectral efficiency than re-using all subcarriersin all parts of every cell (i.e., frequency re-use factor of one). Itmay also reduce the peak throughput to cell-edge users since only asubset of the total number of subcarriers are available for use.

A straight forward extension of the 3G macro-diversity ideas to theseOFDM systems would be to use the cell-edge subcarrier subsets of two ormore neighboring cells to transmit to a MT. The MT would then receivethe same data from multiple BSs (via different subsets of subcarriers)and can combine these to enhance the data reception quality. Thedownside of this approach is of course that this consumes resources(subcarriers) in two or more cells for the benefit of one MT. This isanalogous to 3G macro-diversity, which requires resources (spreadingcodes) to be allocated in two or more cells for the benefit of one MT,and requires the MT to receive and combine two or more signals.

A related piece of prior art is the operation of Single FrequencyNetworks (SFN), which are known in broadcast systems such as DigitalAudio Broadcasting (DAB) and Digital Video Broadcasting (DVB). In theseOFDM systems the same data signal is broadcast from all transmitters. Inthe regions approximately mid-way between two transmitters, thereceiving terminal receives a super-position of the signals from bothtransmitters. This is equivalent to receiving the signal from a singlesource via the composite channel given by the summation of the twochannels from each transmitter. With a suitably long guard interval thereceiver in these OFDM systems can successfully receive the combinedsignal from the two sources with enhanced signal strength over receptionfrom a single transmitter, without Inter-Symbol Interference (ISI), andwithout needing to be ‘aware’ that the signal originated from twoseparate sources.

FIG. 1 shows a schematic block diagram of a transmitter with aspace-time coder 20 adapted to receive an input signal 10 and togenerate two transmission signals 30 which can be jointly received at areceiver.

However, although the SFN concept means that a simple receiver can beused, the combined signal can still undergo fading.

For the case of two transmission paths with respective transferfunctions h₁ and h₂ to the receiver antenna, the combined transferfunction becomes (h₁+h₂), so that the SNR of the received signal is(h₁+h₂)²/n² where n is the amplitude of noise and interference. Butsometimes the particular values of h₁ and h₂ will cancel, significantlyreducing the received power. Therefore the received signal quality couldbe highly variable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide transmission schemewith improved throughput and coverage in cellular systems.

This object is achieved by a method as claimed in claim 1, a computerprogram product as claimed in claim 15, a multi-beam transmission systemas claimed in claim 16, a transmitter device as claimed in claim 20, anda receiver device as claimed in claim 21.

Accordingly, in a cellular system where the same data is to betransmitted to one or more mobile terminals from more than onecell-site, at least one of a space-time and space-frequency coding isapplied to the data from more than one cell-site. This means thatidentical data portions or blocks transmitted from different antennasmay have a different transformation applied by the coder. Thus,increased user throughput at cell edge and increased cell throughput canbe achieved by supporting more active users close to the cell-edge. Thisalso leads to a better coverage.

The substantially different coverage areas may have a non-zerooverlapping area. In the exemplary case of a cellular transmissionsystem, the coverage areas may correspond to different cells of thecellular system. Furthermore, the at least two transmission beams may begenerated by using different antennas at respective different cellsites.

Furthermore, beamformed pilot signals may be used to derive channelestimates for the substantially different coverage areas. Alternatively,if non-beamformed pilot signals are used, beamforming coefficients maybe signaled to the receiver to derive channel estimates for thesubstantially different coverage areas.

The invention may be implemented using concrete hardware units, oralternative as a computer program product, e.g., embodied on acomputer-readable medium or downloadable from network system, comprisingcode means for generating the steps of the above method when run on acomputer device, e.g., provided at a respective transmitter device.

Assuming unity code rate, when a receiving terminal can receive a signalfrom only one antenna, the performance should be the same as for anuncoded system. Similarly, when the receiving terminal can receivesignals from more than one antenna, but they use the same part of thecode transformation, the performance will be no worse than for a singlefrequency network. However, if data with more than one codetransformation is received from different transmit antennas, then thediversity benefit of the space-time or space-frequency coding isachieved.

To support this way of operation it is necessary that the receiver knowsthe timing of the received signals and their channel transfer functions.The timing knowledge can be obtained by transmission of synchronizationsignals from as few as one of the antennas provided the othertransmissions have substantially the same timing (as is required in aSFN).

The necessary channel knowledge can be achieved by associating aspecific (and known), and preferably orthogonal, pilot sequence witheach part of the space-time block code. This allows the receiver to makea channel estimate corresponding to the channels for each antennasending a particular part of the space-time or space-frequency code.

Beamforming may be applied. Then one or more transmissions can beconsidered to be from virtual antennas. The output from a virtualantenna (or beam) may be generated by multiplying a signal by a complexweighting factor (which may be frequency dependent) and transmittingeach of the weighted signals from one element in an array of realantennas. If beamformed pilots are transmitted, then these can be usedto derive appropriate channel estimates. If only unbeamformed pilots areavailable, then in order to derive channel estimates the beamformingcoefficients must be known at the receiver, for example having beensignaled by some signaling unit or arrangement. Different transmissionsfrom different virtual antennas (or beams) may have different parts of aspace-time or space-frequency code applied.

It is not necessarily a requirement to identify which physical (orvirtual) antennas are transmitting which part of the block code.

Further advantageous embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in greater detail based onembodiments with reference to the accompanying drawings, in which

FIG. 1 shows a schematic block diagram of a transmitter with space-timecoder; and

FIG. 2 shows a schematic block diagram of a transmitter with multiplesites, according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described in greaterdetail based on a wireless network environment, such as for example aUMTS LTE network environment.

In wireless broadband systems, the available time, frequency and spatialdiversity can be exploited using space-time codes, space-frequency codesor a combination thereof. As an example, space-time block codes areknown as a way of gaining diversity in systems with multiple antennas. Ablock of symbols is transformed and transmitted from one antenna and thesame data with a different transformation is transmitted from anotherantenna. The concept can be generalized into the frequency domain asspace-frequency block codes or can be extended to cover both time andfrequency. For the known two transmission antenna Alamouti scheme asdescribed for example in S. M. Alamouti, “A simple transmitter diversityscheme for wireless communications”, IEEE J. Select. Areas Commun., vol.16, no. 8, pp. 1451-1458, October 1998, and a single receiver antenna,the received SNR becomes ((h₁)²+(h₂)²)/n². This means that (at least inprinciple) all the received power can be recovered.

According to a first embodiment, an OFDM broadcast downlink is providedin a UMTS LTE network. Each of several cell-sites (corresponding to abase station device or enhanced Node B (E-Node B) in 3G terminology)supports transmissions from more than one antenna, each arranged tocover overlapping geographical areas.

FIG. 2 shows a schematic block diagram of a transmission arrangementaccording to the first embodiment with multiple transmission sites. Aninput signal 10 is supplied to each of a first space-time coder 22 whichgenerates a first part 32 of a transmission signal and a secondspace-time coder 24 which generates a second part 34 of the transmissionsignal.

The transmissions from the same cell-site are synchronized, and thecell-sites are synchronized with each other over an extendedgeographical area. The synchronization may apply at both OFDM symbollevel and frame level.

Furthermore, the same predetermined synchronization sequence (or datapattern) is transmitted from all the antennas of a given cell site.Optionally, different synchronization sequences may be transmitted fromeach cell site. This allows identification of each cell-site. Thesynchronization sequences are designed to have low cross-correlation, sothat they can be transmitted using the same time and frequencyresources.

In a modification, different synchronization sequences may betransmitted from each antenna of the same cell-site. In a furthermodification, the synchronization sequences may be transmitted usingdifferent frequency or time resources. In the latter case the timingoffset should be pre-determined for a given sequence, in order to allowits use as a timing reference.

In a still further modification, different synchronization sequences maybe transmitted from each antenna at a cell site.

Additionally, a pilot pattern may be transmitted from each antenna,wherein pilot transmissions between antennas are orthogonal and have lowcross-correlation. There can be a pre-determined relationship betweenthe pilot pattern transmitted from a particular antenna and the part ofthe block code applied to data transmissions from that antenna. In thecase of the Alamouti space-time block code (for two transmissionantennas) only two distinct pilot patterns would be needed. However,other space-time block codes with different numbers of antennas could beused as well.

In another modification of the first embodiment, the channel estimationis assumed to be carried out using the synchronization sequences, andthere is a predetermined relationship between the synchronizationsequence transmitted from a particular antenna and the part of the blockcode applied to data transmissions from that antenna.

Each antenna at every cell-site is assigned a part of the space-timeblock code (i.e. which would generate a part of the coded output fromthe space-time block coder). This could be such that where coverageobtained from different antennas overlaps, different parts of thespace-time block code are used for those antennas as far as possible.

The data streams from each antenna and cell site are coded according tothe assigned part of the space-time block code. The data may betransmitted in resource blocks extending over more than one OFDM symbolin both time and frequency domains.

At the receiver the following steps may be carried out for eachtime/frequency resource block:

detect one of more synchronization sequences and determine timingreference;

obtain channel estimate(s) for each of the possible pilot patterns (notethat this implies that the number of channel estimates required is notgreater than the number of parts of the space-time block code, with eachchannel estimate comprising an estimate of the composite channel fromall receivable antennas transmitting a given part of the space-timeblock code);

use the space-time block code to decode the data based on the obtainedchannel estimates.

In a further modification, the transmissions from different antennas atthe cell site may have different parts of the space-time block codeapplied.

According to a second embodiment, an OFDM macro-diversity scheme isprovided in a UMTS LTE network.

The second embodiment is similar to the first embodiment, except thatthe transmission is intended for a particular MT, and the signals aretransmitted from a limited number of cell sites (or antennas at one cellsite), e.g., an active set allocated to the MT. The differenttransmissions have different parts of a space-time block code applied.

According to a third embodiment, the proposed scheme is applied to UMTSWideband CDMA (WCDMA) system with beamforming (e.g., by two virtualantennas) and common pilots.

In the third embodiment the Alamouti space-time block code can beapplied and each of the two parts of the space-time block code istransmitted using a different beam (i.e. different virtual antennas).The physical antennas of the beamforming array are located at the samesite. At the receiver the required channel estimates are derived frommeasurements of two orthogonal common pilot signals transmitted fromdifferent physical antennas (not beamformed), together with knowledge ofthe beamforming weights which are transmitted on a separate signalingchannel.

According to a fourth embodiment, the proposed scheme can be applied toa UMTS WCDMA system with beamforming (two virtual antennas) andbeamformed pilot signals.

In the fourth embodiment, the Alamouti space-time block code can beapplied and each of the two parts of the space-time block code istransmitted using a different beam (i.e. different virtual antennas).The physical antennas of the beamforming array are located at the samesite. At the receiver the required channel estimates are derived fromtwo orthogonal pilot signals each transmitted using one of the beams(virtual antennas). As in the previous embodiments, the virtual antennasmay be co-located (at the same cell-site) or not (at differentcell-sites).

In any embodiment it is advantageous if the parts of the space-time (orspace-frequency) code are determined such that the transmitted data canbe correctly recovered by receiving any one of the individual signalscarrying a part of the space-time block code. In this case the totalcoverage area would equal, or more typically, exceed, the union of thecoverage areas reached by each of the individual signals.

In summary, use of space-time block codes or space-frequency block codesin cellular systems has been described. The same data is transmittedfrom different antennas with different coverage areas, corresponding todifferent cells. The different data streams have different parts of thespace-time block codes applied. A MT can combine the different parts ofthe space-time block codes in different received signals. This providesbetter performance than the known techniques for single frequencynetworks. The invention can also be applied to antennas with differentcoverage areas from the same site, and different beams formed withantenna arrays.

It is to be noted that the present invention can be applied to anywireless communication system, particular in cellular systems like UMTSLTE. Moreover, any kind of space-time coding, space-frequency coding orcombined space-time-frequency coding could be used to explore thedesired multi-site diversity effects. The above embodiments may thusvary within the scope of the attached claims.

1. A method of transmitting information in a multi-beam transmissionsystem, said method comprising: a. coding said information by using atleast one of a space-time and space-frequency coding; and b.transmitting the coded information using at least two transmission beamswith substantially different coverage areas.
 2. The method according toclaim 1, wherein said substantially different coverage areas have anon-zero overlapping area.
 3. The method according to claim 1, whereinsaid coverage areas correspond to different cells of a cellulartransmission system.
 4. The method according to claim 3, furthercomprising generating said at least two transmission beams by usingdifferent antennas at respective different cell sites.
 5. The methodaccording to claim 1, further comprising transmitting at least onesynchronization signal via at least one of said at least twotransmission beams.
 6. The method according to claim 5, furthercomprising transmitting predetermined pilot signals allocated topredetermined portions of said coded information.
 7. The methodaccording to claim 6, wherein said pilot signals are orthogonal signals.8. The method according to claim 1, further comprising applyingbeamforming to achieve said substantially different coverage areas. 9.The method according to claim 8, wherein said beamforming comprisestransmitting weighted signals from respective elements of an antennaarray.
 10. The method according to claim 8, further comprising usingbeamformed pilot signals to derive channel estimates for saidsubstantially different coverage areas.
 11. The method according toclaim 8, further comprising using non-beamformed pilot signals andsignaling beamforming coefficients to a receiver to derive channelestimates for said substantially different coverage areas.
 12. Themethod according to claim 1, wherein said transmission is cellularbroadcast transmission, transmissions from the same cell-site aresynchronized and cell-sites are synchronized with each other over apredetermined geographical area.
 13. The method according to claim 12,further comprising assigning to each antenna at every cell-site arespective predetermined portion of said at least one of said space-timeor space-frequency codes.
 14. The method according to claim 1, whereinsaid transmission is directed from a limited number of cell sites to apredetermined terminal device.
 15. A computer program product embodiedon a computer readable medium comprising code means for generating thesteps of method claim 1 and run on a computer device.
 16. A multi-beamtransmission system, wherein information is simultaneously transmittedvia a plurality of beams, said system comprising: a. a coding device forcoding said information by using at least one of a space-time andspace-frequency coding; b. a transmitter device for transmitting thecoded information using at least two transmission beams withsubstantially different coverage areas; and c. at least one receiverdevice for receiving said transmitted information and for decoding itbased on channel estimates and in accordance with said at least one ofsaid space-time and space-frequency coding.
 17. The system according toclaim 16, wherein said transmitter device is configured to assign toeach beam a respective predetermined portion of said at least one ofsaid space-time or space-frequency codes.
 18. The system according toclaim 17, wherein said transmitter device is configured to transmit apilot signal by using at least one of said at least two transmissionbeams and comprising pilot patterns, wherein each pilot patterncorresponds to a predetermined portion of said at least one of saidspace-time and space-frequency coding assigned to the respective one ofsaid at least one used transmission beam.
 19. The system according to16, wherein said transmitter device is configured to use sets ofbeamforming weights, each set corresponding to a predetermined portionof said at least one of said space-time and space-frequency codingassigned to the respective transmission beam.
 20. A transmitter devicefor simultaneously transmitting information via a plurality of beams,said transmitter comprising: a. a coding unit for coding saidinformation by using at least one of a space-time and space-frequencycoding; and b. a transmitting unit for transmitting the codedinformation using at least two transmission beams with substantiallydifferent coverage areas.
 21. A receiver device for receivinginformation via a plurality of beams with substantially differentcoverage areas, said receiver comprising: a. a detecting unit fordetecting at least one synchronization signal and determining a timingreference; b. an estimating unit for obtaining at least one channelestimate based on at least one received pilot pattern; and c. a decodingunit for decoding said information using at least one of a space-timeand space-frequency coding and based on said at least one channelestimate.